It is becoming apparent that in future there will be increased use of new battery systems for both static applications, for example in the case of wind power installations, and vehicles, for example in hybrid and electric vehicles, said battery systems being subject to very great demands in terms of reliability.
The background to these great demands is that failure of the battery system can result in failure of the overall system. By way of example, failure of the traction battery in an electric vehicle results in a “breakdown”. Furthermore, the failure of a battery can result in a safety-related problem. In wind power installations, for example, batteries are used in order to protect the installation against inadmissible operating states in a high wind by virtue of rotor blade adjustment.
The block diagram for a battery system based on the prior art is shown in FIG. 1. A battery system, denoted as a whole by 100, comprises a multiplicity of battery cells 10 which are combined in a plurality of modules 24. Furthermore, a charging and isolator device 12 which comprises an isolator switch 14, a charging switch 16 and a charging resistor 18 is provided. In addition, the battery system 100 may comprise an isolator device 20 having an isolator switch 22.
For safe operation of the battery system 100, it is absolutely necessary for each battery cell 10 to be operated within a permitted operating range (voltage range, temperature range, current limits). If a battery cell 10 is outside these limits, it needs to be removed from the cell complex. When the battery cells 10 are connected in series (as shown in FIG. 1), failure of a single battery cell 10 therefore results in failure of the whole battery system 100.
Particularly in hybrid and electric vehicles, batteries using lithium ion or nickel metal hybrid technology are used which have a large number of electrochemical battery cells connected in series. A battery management unit is used for monitoring the battery and is intended to ensure not only safety monitoring but also the longest possible life. By way of example, a cell voltage sensing unit is thus used.
FIG. 2 shows the known use of such a cell voltage sensing unit.
FIG. 2 shows an architecture which is known from the prior art for typical cell voltage sensing. In this case, each module 24 with its battery cells 10 has an associated cell voltage sensing unit 26. The cell voltage sensing unit 26 comprises a multiplexer 28 which senses the voltage of each of the individual battery cells 10 by using a number of channels 30 which corresponds to the number of battery cells 10. The multiplexer 28 is connected to a gateway 34 via an analog-to-digital converter 32, said gateway being coupled to a communication bus 36. The communication bus 36 has a central controller, for example a microcontroller 38, connected to it. This microcontroller 38 can therefore be used to sense and evaluate the voltages of the individual battery cells 10. The microcontroller 38 may be part of a battery management unit.
As clarified by FIG. 2, a plurality of modules 24 having battery cells 10 may be arranged in series in this case, said modules each having a dedicated cell voltage sensing unit 26.
The multiplexer 28 may also have auxiliary inputs 40, which are known to be able to be used for temperature measurement, for example by virtue of resistance values of what are known as negative temperature coefficient resistors (also NTC resistors) being sensed.
A drawback of the architecture shown is that the central controller 38 merely communicates with the topmost cell voltage sensing unit 26 directly and each cell voltage sensing unit 26 communicates with the one below it in each case. So that all cell voltages can be transmitted to the central controller 38, they need to be passed through all of the cell voltage sensing units 26 which are situated further above. If at least one of the cell voltage sensing units 26 fails, the cell voltages from all of the cell voltage sensing units 26 below it can no longer be sensed by the central controller 38. If, in the extreme case, the topmost cell voltage sensing unit 26 fails, it is not possible for a single cell voltage in the battery system to be sensed.